TLDR: I can't say enough about how much I feel like I learned from having 28-105, in terms of framing + especially in terms of which lens(es) to get next. I'm not sure I would have gotten without that specific lens... and because its still useful and a steal at $350.

I do think that both of them in tandem would cover most (if not all) of the important ranges from 28-105... but I will say that I do think that a person should start with the more affordable options... but also, when first getting a K-1, I do think that its difficult to replace the versatility that exists with the 28-105. Coming from an APS-C kit, I think I learned a lot about how I composed things differently (the 1.5x crop factor does make a pretty big difference), and while you're trying to figure out what/where to go with my next lens... I'm not so sure I would have learned as much if I didn't have that specific lens.

I myself, also really hates buying lenses too that eventually just become obsolete... but even though I have my 24-70 + Tam 90mm... I do keep going back to the 28-105. As a single walk-around I do think that it has value as that. You can make the argument that it is only 30mm, but I would say that there is a lot that you get in that distance. I think out of all of my lenses: Sig 17-35 2.8-4, 35 1.4, 70-200 HSM II 2.8, 150-500 4.5-6.3(?), Pentax 24-70, 28-105, Tam 90mm. If I could only choose one lens to leave the house with, I'd probably grab the 28-105 in most situations (prob 3rd most used). I definitely use it more than my 90 for sure. If I didn't shoot so many of my school events, I'm sure it would be used more than the 70-200. Only if it were night or I knew that the bulk of shooting was done at night would I opt for something else. The only other caveat is that I'd probably never choose it when I do Professional/Events, since light changes so often. I also really like it on my K-3 when I'm double-shooting outside. The 44-157 is really great when paired with the 35 or 24-70 on my K-1.

The limited 70mm 2.4 works very well with the K1. I liked the option of the cropping but also worked fine as full frame. On light trips i would just carry the above and the voigtlander 20mm 3.5.
Probably get both of these for the price of the 77mm limited.

I recently picked up a used K-1 (coming from a K-3 ii) from MPB and got an amazing deal. I've had my eye on the K-1 since it was announced as I love landscape photography and milky way shots.

I've done a tonne of research around FF lenses and I'm a little spoilt for choice it seems. There seem to be a lot of comparisons between various FF lenses such as the FA 50 1.4 and 1.7 but I'm struggling to find comparisons with FF compatible DA lenses like the 50 1.8.

My current lens setup is as follows and I'm trying to work out which to upgrade and which to hang onto:

Sigma 17-50 EX HSM - picked this up as a walk around lens and was really unimpressed with it. Will definitely be selling this as it doesn't cover the FF sensor.

Pentax 50mm f1.8 - easily my favourite lens on the K-3, super sharp and great bokeh. Considering replacing this with a F1.4/1.7

Pentax 35mm f2.4 - slight vignetting on FF wide open but definitely usable. I generally use this as my walkaround as it's far sharper than the Sigma although if I get a 24-70 or 28-105 I doubt I'd get any use out of a 35mm prime.

Samyang 14mm f2.8 - my go to astro/landscape lens. Definitely keeping this one and can't wait to see how milky way shots come out on the K-1.

DA 55-300mm - I don't get too much use out of this so it seems a waste to upgrade. Definitely feels usable on the K-1 either in crop mode or with post cropping in FF. Nice to have for the odd wildlife shot I guess and much lighter than FF equivalents.

With the 50mm being my favourite lens on the K-3 I'm thinking about getting the 77 limited and either the FA 50 1.4 or 1.7 (although I might struggle to pick the 1.7 up in the UK).

My local Jessops/Wex stock very few, if any Pentax lenses so testing them out before buying is pretty difficult.

I'm interested to hear which lenses everyone here thinks I should prioritise and any I've missed! I definitely won't be buying all these lenses straight away as some of them are pretty pricey!

One lens to consider is the DA *55. I think it is better than the FA 50 on full frame. I know it isn't officially listed as full frame by Pentax, but other than weak corners wide open it is pretty good and the FA 50 is worse till f2.8 too. Biggest negative of it is the slow SDM auto focus.

The FA limiteds and the DFA 100 macro are all excellent lenses too that should be considered, particularly if you can find them for a good price used. The FA 35 f2 is a good lens that should be considered if you don't find the corners adequate on your DA 35 and don't want to spend a fortune on the Sigma 35 f1.4 or Pentax FA 31 limited. I wouldn't spend a bunch more on zooms. The DFA 28-105 will cover basic needs and hopefully down the road Pentax will come out with a variable aperture 70-300 zoom that is full frame compatible.

My K-1 kit is basically FA20-35/4, FA43, and something longer like an M135/3.5, M75-150/4 or M80-200/4.5 (I need aperture rings for film cameras). I didn't buy the DFA28-105 as I have loads of old 28-xx zooms and can't justify another. I would like something weather sealed, but don't need anything longer than 43 or 50 until about 135 where I often want something in the 150 or 200 range. I dislike heavy lenses on cameras unless I need longer reach.

Your DA35 and DA50 plastic fantastics will work well enough for starters.
The Rokinon 14mm is a full frame lens, so not problem at all there.
The DFA28-105 is simply brilliant.
For occasional telephoto use, stick with your 55-300 for now. Its weaknesses will become apparent, and their importance will determine how you deal with the problem.